[gimp] app: heed deprectation warning of gegl_operation_context_get_source

[Øyvind Kolås <ok src gnome org>]

commit 9c0dac9e6dd3b3b8994ba1371021ba1db82a3dfc
Author: Øyvind Kolås <pippin gimp org>
Date:   Tue Nov 21 20:21:25 2017 +0100

    app: heed deprectation warning of gegl_operation_context_get_source

 app/operations/gimpoperationhistogramsink.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/app/operations/gimpoperationhistogramsink.c b/app/operations/gimpoperationhistogramsink.c
index aad6757..cfdb80c 100644
--- a/app/operations/gimpoperationhistogramsink.c
+++ b/app/operations/gimpoperationhistogramsink.c
@@ -211,8 +211,8 @@ gimp_operation_histogram_sink_process (GeglOperation        *operation,
       return FALSE;
     }
 
-  input = gegl_operation_context_get_source (context, "input");
-  aux   = gegl_operation_context_get_source (context, "aux");
+  input = (GeglBuffer*) gegl_operation_context_dup_object (context, "input");
+  aux   = (GeglBuffer*) gegl_operation_context_dup_object (context, "aux");
 
   if (! input)
     {